This invention relates to novel bifunctional aromatic cyanates and prepolymers made therefrom for producing thermosetting resins having excellent dielectic properties. Such resins find use in fabricating various electric and electronic parts and electrical laminates in particular. The term "electrical laminates" as used herein refers to unclad insulation boards for mounting various parts thereon as well as metal clad laminates for use in the manufacture of printed circuit boards.
Recently the frequencies used in electronic communication equipments, computers and the like are in a high frequency region such as megahertz (MHz) or gigahertz (GHz) regions. Insulation materials used in such high frequency regions must have a low dielectric constant as well as a low dielectric loss tangent in such high frequency regions. To achieve this a variety of low dielectric constant and dielectric loss tangent resins have been developed. Cyanate ester resins are excellent among others in these dielectric properties. Bisphenol A dicyanate ester resins, for example, have been used in fabricating electrical laminates. Japanese Laid Open Patent Application No. 250359/1988 discloses a fluorine-containing dicyanate ester for producing thermosetting resins having excellent dielectric properties.
The prior art dicyanate ester resins including the afore mentioned resins have excellent dielectric properties compared to epoxy, polyester, phenol and polyimide resins which are conventionally used in the manufacture of electrical laminates. They are, however, not fully satisfactory with respect to dielectric loss tangent in the high frequency regions and in the GHz region in particular. It is desirable for electrical laminates used in the telecommunication field to have a dielectric loss tangent less than 0.005. It is also desirable for the matrix resin of such laminates to have a dielectric loss tangent of less than 0.006, preferably less than 0.005, and more preferably less than 0.004 in the GHz region. For computer application, the matrix resin of electrical laminates should desirably have a dielectric constant of less than 3.0, more desirably less than 2.8, and most desirably less than 2.7 in the GHz region for enabling faster operation.
A need exists, therefore, for a thermosetting resin having excellent dielectric properties and dielectric loss tangent in particular in the UHF region and the GHz region particularly.